1. Field of the Invention
The present invention generally relates to a liquid crystal display device, and more particularly relates to a liquid crystal display device for conducting a display operation using four or more primary colors.
2. Description of the Related Art
A color liquid crystal display device such as a color TV monitor or a color display monitor represents colors usually by adding together the three primary colors of red (R), green (G) and blue (b). Each pixel in a color liquid crystal display device usually has red, green and blue sub-pixels for these three primary colors of RGB. By changing the luminances of these red, green and blue sub-pixels, a variety of colors can be represented.
The luminance of each of the sub-pixels varies within the range from the one corresponding to the lowest gray scale level thereof (e.g., gray scale level 0) through the one corresponding to the highest gray scale level thereof (e.g., gray scale level 255). In the following description, the luminance of a sub-pixel corresponding to the lowest gray scale level will be represented herein by “0.0” and the luminance of a sub-pixel corresponding to the highest gray scale level by “1.0” for the sake of convenience. Therefore, the luminance of each of the sub-pixels is controlled within the range of 0.0 to 1.0.
If the luminance of each of all these sub-pixels, namely, the red, green and blue sub-pixels, is 0.0, the color represented by the pixel is black. Conversely, if the luminance of each of all these sub-pixels is 1.0, the color represented by the pixel is white. Recently, TV sets often allow the user to control the color temperature. In that case, the color temperature is controlled by finely adjusting the luminances of the respective sub-pixels. For that reason, the luminance of each sub-pixel after the color temperature has been controlled to a desired value is supposed herein to be 1.0.
Hereinafter, it will be described with reference to FIG. 26 how the luminances of respective sub-pixels vary in a situation where a conventional LCD changes the colors represented by a pixel from black into white while keeping those colors achromatic. As used herein, the “achromatic” color is a color without a hue such as black, gray and white.
FIG. 26 shows how the colors represented by a pixel change in a conventional LCD as the luminances of respective sub-pixels vary. As shown in portions (a) and (b) of FIG. 26, if the color represented by the pixel is black, the luminance of each of the red, green and blue pixels is all 0.0.
First, the luminances of the red, green and blue sub-pixels are increased at the same rate. As the luminances of the respective sub-pixels are increased, the lightness of the pixel increases and the colors represented by the pixel change from black into gray. In that case, if the luminances of the red, green and blue sub-pixels are increased at the same rate, then the lightness can be increased with the same chromaticity maintained, i.e., with the color represented by the pixel kept achromatic and hueless. If the luminances of the red, green and blue sub-pixels continue to be increased, the color represented by the pixel will change from dark gray into light gray. And when the luminance of each of the red, green and blue sub-pixels finally reaches 1.0, the color represented by the pixel will become white. Conversely, if the luminances of the red, green and blue sub-pixels are decreased from 1.0 to 0.0 at the same rate, then the colors represented by the pixel change from white into black while being achromatic. Thus, a conventional LCD using the three primary colors varies the luminances of the respective sub-pixels at the same rate, thereby changing the lightness of the achromatic colors.
It is known that LCDs have various modes of operation. However, as a TN mode LCD has problems in its display performance (especially in terms of its viewing angle characteristic), various LCDs with improved viewing angle characteristics have been developed recently. Examples of those LCDs with improved viewing angle characteristics include inplane switching (IPS) mode LCDs, multi-domain vertical aligned (MVA) mode LCDs, and axially symmetric aligned microcell (ASM) mode LCDs. Those LCDs operating in new modes that achieve wide viewing angles would not cause problems such as a significant decrease in display contrast ratio when the image on the screen is viewed obliquely and the inversion of display gray scale.
Meanwhile, an LCD that adds together four or more primary colors, not the three primary colors in the conventional LCDs mentioned above, was also proposed. By performing a multi-primary-color display operation with an additional primary color(s) with respect to the three primary colors of RGB, this LCD expands the color representation range (see Patent Document No. 1, for example).                Patent Document No. 1: PCT International Application Japanese National Phase Publication No. 2004-529396        
The present inventors carried out extensive research on a method for getting a multi-primary-color display operation done in a wide color reproduction range by a liquid crystal display device with improved viewing angle characteristic. As a result, the present inventors found the following problems.
Specifically, a so-called “whitening phenomenon” sometimes occurs in a liquid crystal display device that operates in a new mode to achieve a wide viewing angle. As used herein, the “whitening phenomenon” refers to a phenomenon that when the image on the monitor screen is viewed obliquely, portions that should have intermediate gray scale levels look excessively whitish. This whitening phenomenon occurs because the γ characteristic in the oblique viewing direction is different from the one in the frontal viewing direction. That is to say, in these two directions, the γ characteristics have different degrees of viewing angle dependence. As used herein, the γ characteristic refers to the gray scale level dependence of a display luminance. Since the γ characteristics are different in the frontal and oblique viewing directions, the change of the gray scale level (or luminance) varies differently according to the viewing direction. That is why this is a serious problem particularly when a still picture such as a photo is displayed or when a TV program received is presented. If a multi-primary-color display operation were conducted just by adding additional color(s) to the three primary colors used by such an LCD that causes significant whitening phenomenon, the whitening phenomenon would still be quite noticeable and the display quality would never be improved.